Germanium bulk devices and GeOI (“Germanium-On-Insulator”) devices have received much interest in the art as possible replacements for their silicon analogs in some applications, due to the higher electron and hole mobilities in germanium as compared to silicon. For example, it has been shown that germanium-based transistors can exhibit a 400% greater hole mobility, and a 250% greater electron mobility, than silicon-based transistors. This higher mobility offers potential improvements in drive currents that are some 300% greater than the drive currents in comparable silicon devices. In theory, then, it should be possible to make transistors with bulk germanium or GeOI substrates that are much faster than those currently made from bulk silicon or SOI (“Silicon-On-Insulator”) substrates. These faster transistors would enable a variety of technologies, such as the high-speed photodetectors, that are likely to be critical components in future on-chip optical interconnects.
Unfortunately, due to the low energy band gap (Eg=0.66 eV) of germanium, germanium-based transistors suffer from excessive diode leakage currents. In addition, due to the high dielectric constant of germanium (ε(Ge)=16), the junction capacitance (Cj) of these devices is high. Consequently, in practice, it is found that the drive current improvements in transistors made from bulk germanium or GeOI substrates is often compromised by high leakage currents and higher junction capacitance than is found in their silicon counterparts. This issue has precluded the use of these devices in high performance and low power applications.
There is thus a need in the art for a method for overcoming the aforementioned limitations. In particular, there is a need in the art for a method for making transistors from bulk germanium or GeOI substrates that exhibit reduced leakage currents and lower junction capacitance, and for devices made in accordance with this method. These and other needs are met by the devices and methodologies described herein.